1. Field of the Invention
This invention relates transmission tower devices for reducing longitudinal shock loads and particularly to transmission tower devices for reducing longitudinal shock loads using dampers.
2. Description of the Prior Art
Early in the 20th Century, the design and construction of high-voltage overhead electric 10 power lines began. In the nearly 100 years since those first lines were built, the progressive collapse of large numbers of structures has been a continuing problem. These large-scale progressive collapses are known today as “cascades”.
Cascades are typically defined as the progressive collapse of more than two (or three) structures in either direction from an initial failure. In 1916, R. D. Coombs referred to lines “falling longitudinally like a ‘house of cards’”. More recently, there have been a number of major cascades in the U.S. and Canada including 18 steel tower cascades and 37 wood structure cascades in Quebec alone during the great ice storm of January 1998. Cascades can be devastating in their extent. In 1991, Iowa Power lost 108 km of line, including 269 structures, in one cascade; in the 1998 ice storm, 256 structures on Hydro-Quebec's Yamaska to Saint Cesaire line were lost in cascades.
While not all structural failures result in cascades, longitudinal cascades begin with a failure in the structural system that maintains tension in the overhead wires. These failures are represented most simply by a broken wire. Broken wires cause dynamic loads on the towers much higher than the intact wire tensions. Conceptually, the simplest event to trigger a cascade is the tension failure of a cable. Tension failures have been caused by ice overloads, aircraft strikes and damage due to gunshots, aeolian vibration, galloping and electrical arcing during short circuits.
FIG. 1 shows schematically a short section of a typical H-frame line annotated to show the conductors, shield wire, a suspension insulator assembly, a dead-end insulator assembly and tangent and dead-end structures. There are approximately 800,000 circuit kilometers of transmission line 69 kV and above in the United States. Suspension insulator assemblies on tangent structures are perpendicular to the wires and do not support the tension in the wires. At small angles in the line, they will carry the component of wire tension due to the change in direction of the wires. Dead-end insulator assemblies are in-line with the wires and must carry the full wire tension. Most of the structures in a line will be basic tangent structures. The other Structures are used only at the ends of the line and when needed due to changes in the direction of the line or uneven terrain. The problem of preventing cascades centers on the design of the basic tangent structure for longitudinal loads. With intact wires, the tangent structures may see some longitudinal load during construction or when some spans are loaded with ice and other spans are bare. These loads are generally much smaller than those that can occur when a wire breaks. Tests have shown that a tower, which normally is subjected to very small longitudinal loads, can experience very high dynamic longitudinal loads after a failure in the wire tensioning system. These tests have shown that the impact loads on the intact structures from longitudinal disturbances to the wire tensioning system can be as much as four times the residual static loads.
Using special mechanical devices to limit the longitudinal loads due to broken wires and other longitudinal disturbances dates back to the earliest days of transmission line construction. Sliding clamps to limit longitudinal loads were being tried as early as 1910. The use of hinged crossarms was reported in 1928. More recently, crossarms have made purposefully weaker than the rest of the tower to force failures to occur in them rather than in the tower have been used. In addition, bendable link crossarms, insulators with deformable bases, and energy absorption devices have also been used. For example, U.S. Pat. No. 4,791,243 to Ibanez et al. discloses a compact device for long stroke energy absorption that is installed between an insulator and a tower arm. The device consists of two flat steel discs cut so that they will unwind under load into a spiral. However, few of these devices have been successful commercially.
Devices have also be designed to counter “cable galloping” which is a longitudinal disturbance that can occur when ice accretes unevenly on a cable forming an airfoil that can develop lift. For example, rotary friction dampers having a longitudinal V-string of suspension insulators (acting only in tension) to rotate the damper have been proposed to counter galloping. However, such devices are ineffective in countering the impact of broken wire loads that can lead to a cascade.
A need therefore exists for effective means to reduce shock loads on transmission towers during cascades and other longitudinal disturbances. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.